We'll continue our exploration of chord harmony and pop theory by studying 4-note chords. The formulas with which they are derived from the Ionian Major scale are discussed. Modern songwriters don't limit themselves to the use of only these chords in their songs. They will often add chords to the ones they can shape out of the original major scale. We'll take a look at several ways of adding chords to a certain major key and what kind of an effect they generate.